The banner is part of the JBossPortal default skin. You can either download additional skins from the JBoss site, or edit the JBossPortal index.jsp file(s) to modify the skin by hand. The various skins and files can be found in:
jboss\server\default\deploy\jboss-portal.sar\portal-core.war
If you'd prefer not to run in a portal at all, but still want to use JBoss as your app-server, then there are forum articles already posted that explain the steps needed to build/deploy the Alfresco WAR file into a plain (no portal) jboss installation.
Hope this helps,
Kevin